1. Field of the Invention
The present invention relates to a transfer unit incorporated in an image forming apparatus such as an electronic copying apparatus, a printer apparatus and a facsimile apparatus, said transfer unit being for transferring to a transfer sheet a toner image formed on a surface of a photoreceptor incorporated in the image forming apparatus.
2. Description of the Prior Art
In copying apparatuses, the following two types have generally been used as transfer units for transferring to a transfer sheet a toner image formed on the surface of a photoreceptor drum: a non-contact-type transfer unit employing a corona transfer method using a corona discharger; and a contact-type transfer unit employing a bias roller transfer method using a conductive roller.
In a conventional transfer unit of the type employing the corona transfer method, as schematically shown in FIG. 1, a corona discharger 22 is arranged at a necessary distance from a photoreceptor drum 21 rotated in one direction. A transfer sheet P fed into a gap between the drum 21 and the corona discharger 22 is in contact with a toner image formed on a drum surface. By charging the sheet P through the application of a corona charge of a polarity reverse to that of the toner from the reverse surface side of the sheet P, the sheet P adheres to the drum surface, and by Coulomb's force, the toner on the drum surface is transferred to the sheet P.
In a conventional transfer unit of the type employing the bias roller transfer method, as schematically shown in FIG. 2, a conductive roller 23 is arranged to be rotatively in contact with the photoreceptor drum 21 rotated in one direction. The transfer sheet P is pressed onto the toner on the drum surface by the roller 23, and the transfer of the toner is performed by applying to the roller 23 a transfer voltage of a polarity reverse to that of the toner.
In recent years, copying apparatuses and facsimile apparatuses have largely been required to be down-sized. To meet these demands, it is necessary to develop a space-saving transfer unit. However, with the conventional transfer units employing the above-described two transfer methods which require a large space for their paper feeding paths because of its structure, the size reduction of the apparatuses is limited.
Specifically, in the case of the conventional transfer unit of FIG. 1 employing the corona transfer method, since it is difficult to reduce the size of a shield case 22a constituting the shell of the corona discharger 22 because of its structure and since it is necessary to arrange the corona discharger 22 at a distance from the drum 21, it is difficult to reduce the space occupied by the entire transfer unit. Moreover, in this case, since the corona transfer method is of the non-contact type, it is necessary to apply a high voltage to the discharger, which results in the generation of ozone that is harmful to human body.
In the case of the conventional transfer unit of FIG. 2 employing the bias roller transfer method, since it is necessary for the conductive roller 23 to be of a type capable of pressing the sheet P onto the drum surface with no gap therebetween over the entire image area of an axis direction of the drum, the reduction of its diameter is naturally limited. Moreover, since the roller 23 is in direct contact with the drum surface when no sheet is present therebetween, it is readily contaminated by the toner, which necessitates a means for cleaning the roller 23. For these reasons, this transfer unit requires a considerably large space.